This invention relates generally to air or other gas pump operation and control, specifically to the operation and control of a booster (blower) type of pump.
There are many applications in industrial processes and systems wherein it is necessary to evacuate an enclosed chamber to reduce its air pressure a great deal. One such industrial process is the coating of substrates with thin films by sputtering, use of plasma, and the like, which must be accomplished at a very low air pressure. At least a portion of a chamber in which such deposition occurs needs to be opened to the atmosphere around it so that substrates can be moved into and out of the processing chamber. Each time the chamber, or portion thereof, is open to the atmosphere, it must again be evacuated. It is desirable that this evacuation be accomplished as quickly as possible in order to increase the rate at which substrates are coated.
A usual technique for evacuating a chamber in this and other industrial processes and machines is to use a tandem connection of a booster pump (blower) and a mechanical pump. The mechanical pump evacuates the chamber through the booster pump. The purpose of the booster pump is to assist the mechanical pump in evacuating the chamber faster and to a lower pressure than might be possible with the mechanical pump alone. However, the construction of such a booster pump usually compels operating it within limiting operational parameters in order to avoid damaging the pump. A common type of pump is a Roots rotary lobe blower. This type of pump should not be operated with a differential pressure across it that exceeds a certain level, that level usually being established by the manufacturer of the pump. If such a pump is operated for a significant period with a pressure difference that exceeds the recommended limit, damage occurs in the form of seals and/or bearings failing, or by damage to fragile rotating impellers by their hitting the pump's housing. Therefore, in order to avoid costly repairs to a booster pump, with an accompanying down time of the industrial equipment with which the pump is used, such booster pumps are operated within the prescribed pressure difference limit. However, in doing so, the rate in which the chamber can be evacuated is also limited.
One way that is utilized to control the pressure difference across a booster pump is to provide a bypass from its inlet to its outlet that is controlled with a valve. The bypass valve is normally closed when the booster pump is operating in a normal manner but is fully opened to reduce the pressure difference across the pump when operating under conditions that would cause the prescribed pressure difference limit to be exceeded without a bypass. Such a condition occurs when the evacuation of a chamber at atmospheric pressure is commenced.
One specific implementation of the bypass technique (Airco Solar) is to commence such evacuation with the bypass valve open, and keep the valve open until the absolute pressure in the bypass path falls below a limit where, from experience, it is known that a resulting rapid increase in pressure across the booster pump resulting from closing the valve will not exceed the prescribed limit. Once the bypass valve has been closed, it remains closed until the chamber is evacuated to the desired pressure level.
Another specific technique (Pfeiffer) is to delay starting the booster pump until the mechanical pump has drawn the pressure within the evacuated chamber to something less than atmospheric pressure. The booster pump is then operated to join with the mechanical pump in reducing the pressure within the chamber to its desired end point. The booster pump also has a bypass with a relief valve normally closing the bypass. The relief valve opens when the differential pressure across the booster pump exceeds a prescribed limit. The relief valve is a safety device in case the operation of the booster pump otherwise causes the pressure difference across the booster pump to significantly exceed its prescribed limit.
Yet another implementation of the bypass technique (Leybold-Heraeus) also includes a bypass path around the booster pump and a check valve normally closing off that path. As in the immediately preceding described technique, the relief valve is forced open when the booster pump pressure difference exceeds a certain level. The difference here is that when the evacuation of a chamber is commenced, the booster pump is fully operable. This results in the relief valve opening almost immediately upon commencement of pumping of air or other gas from the chamber. But before such a valve is able to respond, the booster pump experiences a sharp, short and high spike of pressure difference which is not desirable. The bypass valve then remains open until the absolute pressure within the bypass path is reduced to a predetermined level at which time it is closed to eliminate the bypass path during the rest of the chamber evacuation process.
Another technique (Edwards), which can be used either with or without such a valve bypass, is to drive the booster pump through a fluid coupling. When the pressure difference across the booster pump increases, the load on its driving motor increases. The fluid coupling allows slippage to occur so that the booster pump slows down, thereby reducing the pressure difference across it. This form of self-correction also occurs when an A.C. non-synchronous electric motor of a direct mechanically driven booster pump is undersized.
It is a primary object ot the present invention to provide an improved technique for controlling the pressure difference across a booster pump in a manner to maintain the wear of the pump within acceptable limits while maximizing the rate at which a chamber may be evacuated of air or other gas.